1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a computer program product for image processing, and more particularly, to those used for correction of a character, a thin line and the like which are present in a halftone dot area.
2. Description of the Related Art
Planographic printing, letterpress printing and intaglio printing customarily use a halftone method to reproduce a gray scale image which has various tints ranging from highlight to shadow. A halftone method is a method which requires to divide a gray scale image into groups of fine elements to reproduce a gray scale in accordance with an area size of fine elements. Various types of forms are actually used as fine elements which constitute halftone. Usually, as in the case of a gray scale image which is used in newspapers, etc., fine elements are fine points, namely, halftone dots, which are arranged with regularity in the form of a matrix at certain pitches in two directions which are perpendicular to each other. In the following, an image reproduced by a halftone method will be referred to as a halftone dot image, and fine elements which form a halftone dot image will be referred to as halftone dots.
When a halftone dot image which is used frequently in printed media is read with an image reader and outputted using a printer, a moiré is generated which has a dependency on a relationship between a spatial frequency of halftone dots and a reading cycle. Further, when image data obtained by reading a halftone dot image are processed through dithering, a moiré is generated which has a dependency on a relationship between a spatial frequency of halftone dots and a dither pattern cycle during the dither processing.
To prevent a moiré generated in such a manner, a conventional digital copier apparatus detects a halftone dot area in an original and smoothes out image data representing the halftone dot area so that halftone dots become indistinctive.
However, when there is a character within a halftone dot area of a print, a conventional digital copier apparatus smoothes out image data which represent the character as well. Hence, the conventional digital copier apparatus can not clearly reproduce the character which belongs to the halftone dot area. Conversely, if the conventional copier apparatus performs edge enhancement on the image data in an effort to clearly reproduce the character which belongs to the halftone dot area, the conventional digital copier apparatus ends up in enhancing even an edge of the halftone dots. This not only fails to prevent a moiré but results in a deterioration in an image quality as well.